1. Technical Field
The present invention relates to an optical disk apparatus and, more particularly, to processing of a reproduction signal output from a split photodetection section.
2. Related Art
A known optical disk apparatus hitherto radiates, on an optical disk, a laser beam of reproduction power from a laser diode of an optical pickup; subjects light reflected from the optical disk to photoelectric conversion performed by a four-quadrant photodetection section of the optical pickup; and adds signals output from respective photodetectors of the four-quadrant photodetection section, thereby acquiring a reproduction RF signal. When the laser beam of reproduction power passes by a pit in the optical disk, there arises a case where a phase difference occurs in output signals of the four-quadrant photodetector stemming from the light reflected by the pit, where some of the output signals are located on a leading side with respect to an advancing direction of the laser beam over pits (hereinafter called simply “leading-side signals”) and the other signals are located on a following side with respect to the same (hereinafter called simply “following-side signals”) The phase difference is ascribable to diffraction induced by the depth of the pit in the optical disk or the pickup optical disk, and changes according to the type of an optical disk apparatus, the type of an optical disk, a reproduction speed, or the like. In relation to the type of an optical disk, in a CD/DVD-ROM signals on the following side tend to lag behind signals on the leading side; in a CD/DVD-RW signals on the following side generally tend to lead signals on the leading side; and in a CD/DVD-R a phase difference tends to hardly arise.
FIG. 4 shows the configuration of a reproduction system of the optical disk apparatus, and FIG. 5 shows the phases of respective signals. In FIG. 4, a pit 6 is formed in a track 5 on an optical disk, and a laser beam 100 of reproduction power is radiated onto the optical disk. A four-quadrant photodetection section 10a detects the light reflected from the optical disk. The four-quadrant photodetection section 10a is divided into four photodetectors; namely, photodetectors A to D. The photodetectors A and B are located at the inner radius of the optical disk, and the photodetectors C and D are located at the outer radius of the same. The photodetectors A, D and the photodetectors B, C are assumed to be split with respect to the direction of the track 5 on the optical disk, and the pit 6 is assumed to move from the right direction of the drawing toward the beam in association with rotation of the optical disk. Under these assumptions, the photodetectors A and D form a group of photodetectors located on the leading side, and the photodetectors B and C form a group of photodetectors located on the following side. A signal A output from the photodetector A, a signal B output from the photodetector B, a signal C output from the photodetector C, and a signal D output from the photodetector D are supplied to an adder 16, where all of the signals are added, and a reproduction RF signal (the signal A+the signal B+the signal C+the signal D) is output. The reproduction RF signal is supplied to the decoder, where the signal is demodulated.
The signals A, D are on the leading side, and the signals B, C are on the following side. There arises a case where the signals on the following side cause a phase difference with respect to the-signals on the leading side for reasons of the type of the optical disk, diffraction induced by the depth of a pit, or the like. FIG. 5 shows a case where the signals on the following side cause a phase lag behind the signals on the leading side. Specifically, the signals B+C on the following side lag in phase behind the signals A+D on the leading side by an amount corresponding to time “t.”
When such a phase lag has arisen, a decrease occurs in the resolution of the reproduction RF signal that is the sum of these signals, and an increase in jitter or deterioration of quality of a reproduction signal is induced during demodulation.
Japanese Patent Laid-Open Publication No. Hei 7-98,938 describes, in an attempt to reduce the distortion of a waveform of a reproduction signal attributable to a frequency characteristic of an optical head, placing at least a pair of photodetectors used for receiving light reflected from an optical disk in alignment with the direction of an information track on an optical disk; placing, at one output stage of the pair of photodetectors, advancement means for advancing a phase of an output from the photodetector in high frequencies; and adding an output from the advancement means and an output from the other output stage of the pair of photodetectors.
However, as mentioned above, the phase difference changes in various manners according to the type of the optical disk apparatus, the type of an optical disk, a reproduction speed, and the like. Hence, the phase difference cannot be compensated for by means of advancing or delaying a phase in an analog fashion by the advancement means, so that the quality of a reproduction signal cannot be sufficiently improved.